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Gianni,

You have produced another magnificent masterpiece worthy of these electronic works of art! I salute you, Sir!

Best regards,

Mike

Quick update: I've begun looking at the scope, and i have found that the + and - 15v rails are shorted, i'll keep looking at it and let you know what i find.

Gianni,

You are truly amazing! You produced a stunningly beautiful draft while troubleshooting a completely foreign instrument.

Thank you for the chance to examine your draft copy. I shall send a summary of my comments tomorrow evening (UTC-8), 1 Dec.

As for hero, I lost that status when my daughter started middle school (grade 6). I remain content with my delusion of adequacy.

Hi,

I recently acquired a 7704A with a similar, if not identical issue.
Since keeping it on for a long time was not a good idea, with scope in OFF state I measured resistance of all power supply outputs; +15V was a nice short. Step by step I isolated the responsible area as the MAIN INTERFACE. Eventually I found C2085 (in perfect looking condition) as the short component. The part list says it's an electrolytic capacitor - in my case it was a Tantalum. I didn't have a Ta and I replaced that with an Al, 100uF/25V, and that solved the issue.
Hope that helps
T

nooo, I am honored to be in touch with one of my heroes! ?

Here you can find the drafts:


(smaller file in lower resolution)

Be merciful and forgive my nonsenses…

Any suggestion/correction is well accepted.

Thanks again

Gianni

Good job!
Of the failures I have had with 465M(1) and 465B(3), failed rectifiers seem to be the most common. Not that I have not had many others - main filter caps, shorted dipped tants, failed diodes, crumbling resistors, shifting resistors, failed transistors (which are a bear on 465M - not socketed - they are socketed on 465 and 465B).
multipliers and transformers holding up so far (I probably should not have written that :-)
phil

I report success.

Turned out to be a blown bridge rectifier, CR721, after all. Which is weird, because it tested ok when I pulled it the first time. I guess it was misbehaving under load, but finally gave up for good while I was testing on the dim bulb current limiter (which saved countless fuses, no doubt). CR721 would up having a <3 ohm short between an AC pin, an DC pin.

In terms of trouble shooting, I first left the secondary windings open, the issue went away, so transformer + filter = good. After reconnecting power to the main board, I pulled the filter caps, which did not help. I then pulled fuses F736, F746, and F768; the issue remained, but now I knew it had to be a rectifier, a ceramic cap across the rectifiers, or less likely, a resistor off those rails. After pulling CR721, the issue abated. Hopefully this helps someone in future.

I have placed what I had on hand in as a replacement (2A, 600V), and all is back up an running. I will change the other rectifier, and main filter caps, as soon as I do my next Mouser order.

Thank you all for helping me out. This forum is fantastic. Cheers.

Nick

There are diodes in the d-c restorer circuits that will go bad causing the high brightness. Replace them with 1N4937. The two 22meg resistors should be replaced with Vishay VR37 types.

The Z board has resistors that should be increased in wattage. The 3600 ohm 2W should be 7W. This resistor is close to a pot that can get burned enough to make it bad.

The condensers should be replaced. Nichicon ULD, UCY and UHE types are best. The LGR can be used to restuff the two screw mount cans in the power supply.

Mark

The A version still uses them on the switching PS boards as well as most, if not all, of the other boards. In my latest 7704A, I had one tantalum catch fire on the horizontal amplifier board while I was distracted with something else. Then one on the main interface board smoked quite a bit before I could react quickly enough to turn the scope off.

BTW, if you do find a low/missing rail, there are five caps on the main interface board (C2081 C2083, C2085, C2087, and C2089) that often go very leaky or short entirely and some (all?) of those are tantalums. I think that arrangement is somewhat common on many of the 7000-series scopes.

Any of the above can be replaced with good-quality electrolytics.

Barry - N4BUQ

Well, that's both good and bad things showing. The good parts are that's probably why it was free, and the high voltage system is probably OK, leaving hopefully only minor, simple problem(s) like low a voltage supply failure. I don't know whether or not the more modern "A" versions of these scopes totally eliminated tantalum capacitors for power supply bypassing throughout the scope. I'd guess there are still a few in certain spots, so they are the prime suspects. Without more info, I'd guess either the plus or the minus 15V supply is dumped by a shorted big Ta cap on the main interface board - those are spots where I think Ta caps may still be used. Whatever it is, it's not serious enough to totally crash the supply, or it wouldn't light up the screen.

Once you get the manual and info you can start looking. If you're not aware of the Ta caps and other common problems, ask about it, and you'll get plenty of info and history from the group. Good luck.

Ed

Lucky...

Inability to control the intensity most likely indicates a problem with the Z-Axis circuitry. Note that the Z-Axis board depends on signals from the Logic Board so there are a few places to look for problems. If you can get it to where you have control over the intensity, then you can start looking for other issues.

Have you checked to ensure all the power rails are there and within spec?

Do you have another scope so that you can check for signals on the various boards?

Good luck,
Barry - N4BUQ

Hi there, i recently found a free tektronix 7704A Oscilloscope, but unfortunately it isn't working properly. When i power it up the only thing that shows up is a very bright (so bright i suspect it would damage the phosphor if i left it on too long) Spot, which only changes shape with the Focus and Astig Controls. All other controls do nothing and it's the same regardless of whether the acquisition modules are installed. Does anyone have any suggestions for what to look at? I'd love to get this thing working!


Here's a photo of what i'm seeing: /g/TekScopes/album?id=299173

I will be honored with a copy.

During the 1130x development, I was told that I had psychic debugging skill. It seems to work over a great distance too!

Thanks! Without your intuition about the intensity In would not succeed.

Interesting: I thought that a strange behavior in the brightness control was a fault of my gear. I order the new EPROMs and see what it changes.

If you like, as soon as I have a draft for the new book with the 11302, I make available a pdf for you.

Greetings from Florence, Italy

Gianni

That is great news! I am happy that you were able to restore EA function and save another scope.

If you are able to update the firmware to 3.1, you will find significant improvements. Version 2.7 was a very clean release with no known bugs, as I recall. Version 3.1 is a feature upgrade.

Firmware version 3.1 includes many features that are available in the A-version such as histogram-based measurements(never done in an analog scope) and a one-touch shortcut to set up for delay time measurements with the counter-timer.
The most noticeable improvement is the intensity control. The early FW releases (2.7 and below) used a very complex algorithm to compute variables to manage physics of the CRT and the different intensity levels of text characters, xy trace, main trace and delayed trace. This is a messy task to make each appear independent when they all use the same MCP image intensifier. The extensive calculations required too much time and make the intensity control annoying to use. Version 3.1 is a big improvement.

I brought back from the farm my 7603 to see the physical items and layout involved, and also found I have a bunch of saved parts from junking a couple 7603s long ago. The R7613 transplant looks like it would have some big mechanical issues, especially in the length of the CRT shield and its mounting. I'm thinking now of trying to make another 7603 from the 7403N carcass, which should be fairly easy, IF there are enough 7603 parts still good. If not, then at least I'll have many of the items common between the 7603 and 7613 worked out, and will be able to see how much more is needed for the complete conversion.

Ed

EVVIVA! (typycal Italian expression of great joy)

I was closing the email below, when I remembered something I read in the 11302 Firmware Replacement Technical Bulletin that very kindly Hakan Hintze sent to me with the 3.1 upgrade eproms. That sounded like "If the instrument fails to do a successful EA, run the GRAT-PREP procedure, Test List No. 24 in the CATS disk”.
This procedure allows to tune the screen alignment procedure and is really well done; it as formerly named in the manuals as “19 .BEAM_REGISTRATION”. It allows to set the scanning marks position and intensity and allows to see what the photodiodes “see”. I carefully performed it and …it works!
Thanks to everybody, without your help I could not succeed!
Gianni
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This oscilloscope is very complex and the first approach is disorienting. After several days with my brain inside it and in the manuals, perhaps I begin to understand something. The baby has at least three different problems, and I could fully understand just one, very simple:

1) the “stupid" problem: the dip switches are faulty and betray you blatantly - a stupid fault which creates great confusion. I temporary fixed them but I will replace them;

2) the “main” problem: the Enhanced Accuracy most of the times fails.
The failure: The Enhanced Accuracy fails with the message “Mainframe amplifier calibration failed”.
Early fail - When it fails, it fails always in the first stages. When it arrived to phase of “E” symbol in the attached diagram, it always complete the EA;.
When it works? But when does it succeed? In a world: at cold.
Success: When I do the EA immediately after power on, subsequent to a period off (20 minutes off or more). In these conditions the markers on the CRT the calibration shows are much brighter, with a halo (and EA works). In this way, I could even do positively two consecutive EA. But fe minutes later the EA fails.

3) enlarged screen - from time to time the screen get badly shifted to the right and / enlarged vertically. Perhaps it is in consequence of a failed EA, but not always.

4) all the extended tests pass, even looping.

5) optical issues - I dismounted the front panel and checked everything. I cleaned the four photodiodes with isopropyl (but tey were clean). On the colored optical filter, I found a small abrasion just in the area interested by the beam scanning photodiodes. I tried to remove it with toothpaste but I could only worsen the situation. So I remounted it facing outside. But @Clark, I believe that you had a good intuition, as I explain below.

6) documentation mismatched - this is one of my biggest problem, because the TekCATS is much newer than the manuals I could find and the tests are very different, so I don’t have a description for the various tests.

7) standardizers - I have no standardizers, so I try to do what I can. But there are tests, like vertical gain, where I could not find a workaoround.



BUT SOMETIMES IT WORKS! If I a) I keep it off for several minutes; b) switch it on and try the EA, in most of the times it works and the EA is correctly performed. And what I observe enforces the Clark’s optical theory.

I noted that I can predict the result:
when the EA will succeed, the calibration marks are very bright, with a marked halo;
when the EA will fail, the calibration marks are not very bright.

The brightness of the calibration marks does not dependent from the front panel settings, but is preset by the system. I think that it is an analog problem from the CRT circuit.

It took 2 versions as I am learning CNC/CAD. jlcpcb. Way way cheaper then north american shops of course.
There are issues in my design but - good enough for now.

That's awesome! Who did the CNC work?

-- john, KE5FX

Thanks for your suggestions, I work on them and update.